Casting mold and method of preparation



Jul)'19,1960 N. HERZMARK ETAL 2,945,273

CASTING MOLD AND METHOD OF PREPARATION Filed April 2a, 1958 CASTING MOLD AND METHOD OF PREPARATION Nicolas Herzmarlr, Villa la Rustica, Clarens-Montreux, Switzerland, and Auguste Leschot, Paris France; said Leschot assignor to said Herzmark The invention relates to improved refractory jointless shell molds for investment casting, and to a method of preparing such molds.

Such molds are generally made by applying to the surface of a destructible or fusible pattern a slurry of a finely divided refractory material with a binder, which consists mostly of organic silicates, aquaor organosols of silica, or acidified inorganic silicates. Such molds should combine sufficient strength for withstanding mechanical stresses, particularly the pressures produced by pouring the metal to be cast, with a certain porosity allowing escape of water vapor and gases so as to avoid cracking of the mold during manufacture and use. The conventional procedures for the preparation of investment molds have not yet solved satisfactorily the problem of imparting to molds simultaneously optimum strength and porosity, because said two properties appeared to a certain extent to be mutually exclusive.

It is a principal object of the invention to provide refractory shell molds for investment casting which combine a high degree of strength and hardness with sufiicient porosity and elasticity to avoid development of internal stresses and to allow diffusion of gases and vapors so as to prevent cracking on heating and during the casting operation.

It is another object of the invention to provide a meth: od for preparing such refractory shell molds.

Other objects and advantages will be apparent from a consideration of the specification and claims;

. According to the invention, the shell mold is built up from several layers,'whereby the inner layers provide the required porosity and elasticity and the outer layers impart the necessary strength and rigidity. We obtain such differentiation by converting a major portion of the used potassium or sodium silicate binder in the inner layer to a silica gel while the binder in the outer layers is essentially silicate which has been insolubilized by heating.

The invention will be described more in detail with reference to the accompanying drawing, in which:

Fig. 1 is a diagrammatic View, partly in section of a mold according to the invention built up around a cluster of patterns attached to a main sprue;

Figs. 2 to 6 illustrate diagrammatically on an enlarged scale the gradual build-up of the mold by applying tothe patterns successive alternate layers of refractory coatings and sandings, whereby the four outer refractory coatings are treated differently from the preceding inner coatings.

Referring to the drawing, the reference numeral 1 designates a preformed refractory or metallic block, carrying a main sprue 2 attached to the patterns 3. For pun poses of illustration, said patterns are shown as simple cylindrical bodies with a cylindrical stem.

n the pattern 3 of wax or other readily fusible or destructible material, a first layer d of a refractory slurry (Fig. 2) is formed. Generally, such slurry is prepared by combining two diiferent refractories with an alkali metal silicate binder.

thermal shock treatment has the efiiect of removing 2345273 Patented July 19, 1960 Said slurry is applied by spraying or. dipping in such an amount that the thickness of the layer is about 0.2 to 0.4 mm.

The coated patterns are then sanded with a coat e of a dry refractory such as silica sand of to mesh whereby the thickness of the layer is increased to about 0.7 to 0.9 mm.

The entire cluster is then contacted with an acid atmosphere for a short period of time, for instance one minute or less, whereby the gaseous acid reacts with the silicate to, form the corresponding sodium salt and free SiO This treatment hardens the coating and renders it suitable for the subsequent application of a second slurry layer d and sand layer e which are treated with a gaseous acid in the same manner as described for the first layers. This procedure is repeated until a mold of a desired thickness, for instance, 4.5 to 6 mm, has been built up; in the mold shown in the drawing, seven layers (slurry+sanding) were used (Fig. 6).

The rapid insolubilization of the successive layers provides for 'a firm anchoring of each layer in the preceding layer.

As acid, we prefer to use carbon dioxide, but other acid vapors, such as hydrochloric acid, may also be applied. In the case of carbon dioxide, the silica gel binder is formed by the reaction Preferably, such an amount of acid should be reacted with the alkali metal silicate that the mol ratio of silica gel formed to remaining alkali metal silicate in the treated layers is about 2 to 5:1.

It-is an essential feature of the invention that the acid treatment is not applied to the last layer or layers where-. by the number of untreated outer layers will depend on the strength and hardness of the mold required for the particular casting process used. Generally, it will be suficient to omit the acidfying treatment of the outermost or the last two to four layers and to replace said treatmentby drying instead each of said layers in a current of air at a temperature of about 40 to 45 C.

Said air drying operation converts the layer within 30 to 40 minutes to a very hard rigid structure of reduced porosity, whereby the sodium silicate is insolubilized.

Still lower temperatures could be used but are not suitable for commercial production because of the required longer drying times.

The gentle drying below 45 C. avoids any harmful expansion of the pattern material, thereby preventing the development of cracks in the mold under construction; at

the same time, it effects a slow continuous elimination of the Water contained in the inner porous mold layers Instead of said gentle drying step, other procedures may be used to remove part of the water from the outer layers. For instance, after applying each of the outer nonacidfied outer layers and sanding, the mold may be dipped into an inflammable organic solvent for water, such as alcohol, acetone, or dioxane. The mold is rapidly taken out of the solvent and lighted so as to burn ofi the solvent.

With inflammation of the solvent, also part of the water is evaporated, and at the same time the silicate binder is insolubilized. As there are between the burning layer and the wax pattern the acidified moisture-containing layers, the patterns are protected from the heat developed by the burning solvent.

In still another modification of our invention, the outer non-acidified layers may be dried by subjecting the mold for a short time of about 30 to 60 seconds to a temperature of about 250 to 850 0, whereby the higher temperatures correspond to shorter heating times. Such quickly a major part of the water contained in the respective outer layer and thus polymerizing the silicate and insolubilizing the layer. As also in this case the wax patterns areprotected by several acidified layers containing .a certain amount of moisture, the short heat shock will not produce premature melting of the pattern. The heating time and temperature will depend on the surface of the mold; the larger the surface, the shorter will be the heating time to avoid the penetration of the heat beyond the thickness of the treated layer.

All the described heating procedures for the nonacidified layers need be applied only to the layers below the outermost layer since the outermost layer will be automatically insolubilized during the drying operation proper and the elimination of the wax pattern.

The completed moldconsists of an inner and an outer shellportion whereby the major part of the silica binder in the inner shell portion is present in the form of'silica gel and in the outer portion in the form of insolubilized alkali metal silicate; the thicknessof the inner shell portion should be about 4 to 4 times that of the outer shell portion. The mold is then subjected to a drying operation, Where slowly increasing temperatures are applied. We have found it of great advantage to carry out the first drying step at a temperature of 35 to 45 C. because at said temperature the viscosityof the water decreases by 50 percent, which increases considerably the rate of diffusion of the water contained in the mold layers to the outside. After about 25 percent of the total water has been thus removed, the temperature may be increased rapidly until the patterns of fusible material have been molten and withdrawn through the sprue. As an example, the drying cycle may be as follows: 3 hours at 40 C., 2 hours at 50 C., Zhours at 60 C., and then heating at 100 C. until the wax patterns have been melted out. In this procedure, a total amount of .about 40 .to 50 percent of the total water contained in the coatings should be removed at temperatures not exceeding about 60 C.

In cases where the outer layers have beendried by the described solvent or heat shock treatment, the gradual pro-heating of the molds maybe omitted and the molds may be heated directly to atemperature of 80 to 90 C. to melt the patterns. The remaining moisture is quickly removed at said temperature-whereupon the molds are ready for the final heat treatment described hereinafter.

Subsequently, the shell moldsstill adhering to the refractory block are placed in a dryingoven for complete drying and removal of any organic substances. While in the conventional preparation of shell molds it is generally necessary to increase the temperaturegradually from about 200 to 1000 C.'over a period of 12m 22 hours to prevent cracks and deterioration of the molds,- molds prepared according to the invention may be heated directly without injury at about 800 to 900, and are. ready to receive the casting metal after a heating time of only about 1 hour. V

The following example is given to illustrate the invention.

A wax pattern was dipped in aslurry of the following composition:

Parts'by weight .Sodium silicate 9 Water 11 Silica sand (80 to 100 mesh) '20 Calcined alumina (200 mesh) wherebya coat of 0.3 .mm. thickness was applied.

The coated pattern was dusted with silica. passing a 1.00 mesh sieve, thereby increasing the thickness of the layer to 0.7 mm., and then exposed for-50 seconds to the action of. carbon dioxide. The dipping, sanding, and carbon dioxide treatment were repeated 3 times; subsequently a fourthlayer was applied by dipping andsanding, .butthistimethe mold was not exposedto carbon dioxide but heated for 30 minutes in air of about 42 C., whereby about 25 g. of water were evaporated. A fifth and sixth layer were applied in the same manner, and the mold was once more dipped and sanded to apply a seventh layer as the last layen The heating insolubilizes'thesodium silicate and prevents its dissolution on application of the next layer.

With each of the seven layers, water in a total amount of g. (free and chemically boundlhad been brought in, .of which 25 g. had been evaporated each time when one of the outer layers was heatedat 42 C. After the sanding of the last layer, the mold, which contained still 625 g. of water, was air-heated for 3 hours first at 40 0, thereby removing 100 g. of water; then the temperature was increased to 50 C. and maintained for two hours, thereby evaporating .80 g. ,of water, and 30 more grams of water were removed in the next 2 hours by heating at 60 C.

In this way, almost 50 percent of the total water was removed at temperatures not exceeding 60 C.

The mold was then heated to about 100 C. to melt the wax, and after removal of the wax it was fired, withoutany external support, for only 50 minutes at 900 C., whereupon it was ready for casting, which was also carried out in the unsupported shell.

,As the molds according to the invention comprise an elastic and porous inner gel portion and ahard rigid outer portion .of insolubilized silicates, -the invention allows of adjusting the properties to the desired requirements by adjusting the relative thicknesses of said inner and outer portions. In addition, the very short manufacturing cycle considerably'simplifies the manufacture of the shell molds and renders our process suitable for automatic mold production.

It might be noted that it has already been proposed to prepare molds from refractory materials with a liquid silica binder by precipitating silica therein by means of a gaseous acid. However, such molds could not be used to any large-extent because they did not have the required high hardness and breaking strength.

It will be understood that we do not intend to be limited to the exact details and compositions as described; such details and compositions may be'varied within the scope of theappended claims.

We claim:

1. A shell mold for investment casting consisting essentially of a comminuted refractory material and a silica binder for said refractory material, said mold comprising at least one inner layer in which the major portion of said binder is present in the form of silica gel, the balance being alkali metal silicate, and at least one outer layer containing the binder essentially in the form of alkalimetal silicate, said inner layer imparting to the mold porosity and elasticity and said outer layer imparting strength and rigidity.

2. A shell mold as defined in claim 1 wherein thickness of said inner layer is about %-4 times thickness 'of said outer layer.

3. A shell mold as defined in claim 1 wherein the ratio silica gel2alkali metal silicate in said inner layer is about 2 to 5:1.

4. A shell mold as defined in claim 3 wherein said inner layer contains sodium carbonate in a mole amount substantially corresponding to said silica gel.

5. A shell mold as defined in claim 1 wherein said alkali metal silicate is sodium silicate.

6. A 'method of preparing a jointless'shell suitable for investment casting comprising repeatedly coating a fusible pattern with an aqueous slurry containing comminuted refractory material and an alkali metal silicate binder, treating each coat with an inorganic gaseous acid for a time sufiicient to convert at least half the amount-of said alkali metal silicate binder into a silica gel and the alkali metal salt of said acid, applying to the thus coated pattern additional outer coatings of said the the aqueous slurry while discontinuing said treatment withan inorganic gaseous acid, partially dehydrating and thereby insolubilizing the alkali metal silicate binder individually by a heat shock treatment of each of said additional coatings, and removing the fusible pattern, said first applied silica gel containing coatings providing a porous inner shell portion of the mold and said insolubilized alkali metal silicate containing coatings providing a hard rigid outer shell portion of the mold.

7. The method as defined in claim 6 wherein said 10 gaseous acid is carbon dioxide.

8. The method as defined in claim 6 wherein said outer coating is insolubilized by a shock heat treatment at a temperature of about 250 to 850 C. for a period of time of about 30 to 60 seconds, the shorter times be- 1 ing employed for the higher temperatures.

9. The method as defined in claim 6 wherein said insolubilization of the outer coating is effected by contacting said coating with an inflammable water-miscible organic solvent and burning off said solvent.

References Cited in the file of this patent UNITED STATES PATENTS 2,806,270 Shaul Sept. 17, 1957 FOREIGN PATENTS 712,276 Great Britain July 21, 1954 716,394 Great Britain Oct. 6, 1954 742,361 Great Britain Dec. 21, 1955 774,184 Great Britain May 8, 1957 787,052 Great Britain Nov. 27, 1957 

1. A SHELL MOLD FOR INVESTMENT CASTING CONSISTING ESSENTIALLY OF A COMMINUTED REFRACTORY MATERIAL AND A SILICA BINDER FOR SAID REFRACTORY MATERIAL, SAID MOLD COMPRISING AT LEAST ONE INNER LAYER IN WHICH THE MAJOR PORTION OF SAID BINDER IS PRESENT IN THE FORM OF SILICA GEL, THE BALANCE BEING ALKALI METAL SILICATE, AND AT LEAST ONE OUTER LAYER CONTAINING THE BINDER ESSENTIALLY IN THE FORM OF ALKALI METAL SILICATE, SAID INNER LAYER IMPARTING TO THE MOLD POROSITY AND ELASTICITY AND SAID OUTER LAYER IMPARTING STRENGTH AND RIGIDITY. 